This patent application is related to U.S. patent application Ser. No. 10/428,261xe2x80x9cunassignedxe2x80x9d filed concurrently herewith and entitled xe2x80x9cOptical Interference Display Device,xe2x80x9d which is herein incorporated by reference.
The present invention relates to the field of micro-electromechanical devices. More particularly, the present invention relates to charge control of a micro-electromechanical device.
Micro-electromechanical systems (MEMS) are systems which are developed using thin film technology and which include both electrical and micro-mechanical components. MEMS devices are used in a variety of applications such as optical display systems, pressure sensors, flow sensors and charge control actuators. MEMS devices use electrostatic force or energy to move or monitor the movement of micro-mechanical electrodes which can store charge. In one type of MEMS device, to achieve a desired result, a gap distance between the electrodes is controlled by balancing an electrostatic force and a mechanical restoring force. Digital MEMS devices use two gap distances, while analog MEMS devices use multiple gap distances.
MEMS devices have been developed using a variety of approaches. In one approach, a deformable deflective membrane is positioned over an electrode and is electrostatically attracted to the electrode. Other approaches use flaps or beams of silicon or aluminum which form a top conducting layer. With optical applications, the conducting layer is reflective and is deformed using electrostatic force to scatter light which is incident upon the conducting layer.
One approach for controlling the gap distance is to apply a continuous control voltage to the electrodes, wherein the control voltage is increased to decrease the gap distance, and vice-versa. However, this approach suffers from electrostatic instability that greatly reduces a useable operating range over which the gap distance can be effectively controlled. This is because the electrodes form a variable capacitor whose capacitance increases as the gap distance decreases. When the gap distance is reduced to a certain threshold value, usually about two-thirds of an initial gap distance, the electrostatic force of attraction between the electrodes overcomes the mechanical restoring force causing the electrodes to xe2x80x9csnapxe2x80x9d together or to mechanical stops. This is because at a distance less than the minimum threshold value, the capacitance is increased to a point where excess charge is drawn onto the electrodes resulting in increased electrostatic attractionxe2x80x94a phenomenon known as xe2x80x9ccharge runaway.xe2x80x9d
This non-linear relationship between the control voltage and the gap distance limits the controllable range of electrode movement to only about one-third of the initial gap distance, and thus limits the potential utility of the MEMS device. For example, with optical display systems, interference or detraction based light modulator MEMS devices preferably should have a large range of gap distance control in order to control a greater optical range of visible light scattered by the optical MEMS device.
One aspect of the present invention provides a charge control circuit for controlling a micro-electromechanical system (MEMS) device having variable capacitor formed by a first conductive plate and a second conductive plate separated by a variable gap distance. The charge control circuit comprises a switch circuit configured to receive a reference voltage having a selected voltage level and configured to respond to an enable signal having a duration at least as long as an electrical time constant of the MEMS device, but shorter than a mechanical time constant of the MEMS device, to apply the selected voltage level across the first and second plates for the duration to thereby cause a stored charge having a desired magnitude to accumulate on the variable capacitor, wherein the variable gap distance is a function of the magnitude of the stored charge.